Round vs Euclidean
- Thread starter delsi
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gordo
Well-known member
This has been asked before...but basically:
In a CTP environment the best AM/XM dot shape is round. By that I mean a non-transforming dot that is round in the highlights and simply increases in size to represent darker tones. Here are a few reasons why this is the "best" dot shape for CTP compared to Euclidean, Elliptical, Square, etc. dot shapes:
1) Dot shape is the same for all screen angles - eliminates dot gain due to dot shape variables
2) Reduces single channel moiré issues
3) Dot is non directional, i.e. all screen angle dots react the same to directional press issues such as slur and doubling
4) The tone bump that occurs when dots touch occurs in the shadows at 75% so is much less visible in presswork (e.g. vignettes/skin tones) compared to Euclidean (50% bump where dots form a checkerboard) or Elliptical 40% & 60% bump.
Round dot is not normally used in a film workflow because, with linear film, the dots touching in the 3/4 tones results in a loss of shadow detail.
best gordo
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meddington
Well-known member
We use stochastic screening primarily, but out of the two, I would personally opt for euclidean over round dot. As tone value increases and the round dots meet, there can be a tone jump in the gradient. Euclidean dots form a checkerboard pattern at midtone, and as dots meet there is less area imparting a tone increase.
Also, the checkerboard pattern is a dead givaway to the true 50%.
Also, the checkerboard pattern is a dead givaway to the true 50%.
meddington
Well-known member
For the record, I posted before reading Gordo's response, and I'm reluctant to argue against his advice.
But...
... with euclidean's checkerboard pattern, the elongated axis incurs the tone bump first, and the second axis occurs a bit higher up in the tone scale, making this not such an abrupt transition, no?
But...
... with euclidean's checkerboard pattern, the elongated axis incurs the tone bump first, and the second axis occurs a bit higher up in the tone scale, making this not such an abrupt transition, no?
gordo
Well-known member
For the record, I posted before reading Gordo's response, and I'm reluctant to argue against his advice. But...
... with euclidean's checkerboard pattern, the elongated axis incurs the tone bump first, and the second axis occurs a bit higher up in the tone scale, making this not such an abrupt transition, no?
No.
I think you're confusing Euclidean with Elliptical.
Euclidian is round/square/round(inverted) The dot shapes are non-directional - there is no elongated axis.
Elliptical dot shapes are diamondish ellipticalish. They have an elongated axis.
Here's what Euclidean looks like close up. This sample is Agfa Balanced Screening 150 lpi Euclidean dot (it's from some screening work I did with them a few years ago)
The big problem with Euclidean, as I mentioned, is that the round dots become square - a checkerboard - when they go through the 50% tone. This causes an "optical bump" - a visible dark line at the 50% tone - when the four corners of the dot all touch at the same time. It's especially visible in vignettes.
BTW - Agfa uses a neat trick to try and minimize this issue. You can see it if you examine the black printer up close. (ignore the extra color dots you'll see they are caused by a color management error)
Elliptical mitigates this by splitting the point where dots touch to the 40 and 60% tone value - but introduces other issues - some of which I mentioned before.
Round dots put the optical bump at the 75% tone - i.e. deep in the shadows where it is less visible.
Here's a link to an animation of an elliptical screen to make it easier to understand: http://qualityinprint.blogspot.com/2008/12/150-lpi-elliptical-halftone-dot.html
RE: "Also, the checkerboard pattern is a dead givaway to the true 50%"
To form a perfect checkerboard at 50% requires that the halftone frequency (lpi) be an even divisor of the dpi of the imaging device as well as being image with a non-gaussian laser - which seldom happens in the real world.
Glad you're using FM screening though!
and PeterA wrote:
That person was likely Dan Gelbart and was trying to demonstrate SquareSpot imaging technology - not halftone screening. Laser spot vs halftone dot. You're not the only one who had a difficult time understanding the difference - or its significance.
best, gordo
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VladCanada
Well-known member
He-he... Square laser super-pooper dot. Old song of Creo-Kodak.
A lot of lances were broken in the battle of discussions.
A lot of lances were broken in the battle of discussions.
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gordo
Well-known member
I'm not an apologist (nor singer) for Kodak
. Suffice it to say that I believe (as did engineers such as Dan Gelbart) that the more a prospective customer understands the processes related to their work (e.g. Print manufacturing) as well as the technology behind the tools available to them (e.g. CTP technology), then the better able they will be at making the correct choice for their business.As the slogan I created for the creo user's association says "Knowledge is power, knowledge shared is empowerment"
On the other hand, a realization I experienced when I taught graphics many years ago is that: You can't teach people anything - it is up to them to learn.
best, gordo
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Stochastic /Stacatto
Stochastic /Stacatto
During the last 4 years I have imaged about 6 - 8000 B2 plates a month using Stacatto and printed them to ISO (Fogra really) and not had any problems matching proofs, hitting ISO number etc - why are you still talking about conventional screening
Stochastic /Stacatto
During the last 4 years I have imaged about 6 - 8000 B2 plates a month using Stacatto and printed them to ISO (Fogra really) and not had any problems matching proofs, hitting ISO number etc - why are you still talking about conventional screening
PrepressDan
Well-known member
Gordo, have you ever run Concentric screening from Artwork Systems? We are getting ready to try it out. I love what I have seen with some of there customers inviting us for a tour.
meddington
Well-known member
Some may not want to pay the premium for the priviledge?
gordo
Well-known member
Yes, I'm quite familiar with concentric screening.
Since it is at its core an AM screen - you should test/compare it against a conventional screen at the same frequency (lpi) I.e. Concentric vs Round dot at 240 lpi (or whatever) You may find that there is no visible, or measurable, difference in your presswork when you do that. However, Concentric does demand a great deal from your CtP system because it acts like a frequency multiplier, single pixel rings, used to build a halftone dot are as hard to image consistently as a 10 micron FM screen (also single pixel dots). You will likely need to define a frequency and ring thickness specific to your shops condition to make it work. That is if you will be printing around 150-175 lpi or so.
IMHO, if you want to stay with an AM screen rather than go FM to go to higher resolution (i.e. over 240 lpi), I would stick with a conventional AM or XM type screen.
The technology of punching holes (of one sort or another) in AM dots to either reduce ink consumption, increase gamut, increase press side stability, or provide anchors for ink in, for example flexo printing is well known (and patented). So it's worth testing out. Just make sure your test is rigorous, valid, effective, and compares apples to apples.
PS I forgot to include a link to an animation of an elliptical AM screen in my previous post - here's the link:
Quality In Print: 150 lpi Elliptical Halftone Dot
best, gordo
gordo
Well-known member
Yup. And often printshop culture is the first barrier (why fix it if it ain't broke mentality) to changing anything - especially screening.
gordo
otherthoughts
Well-known member
Perhaps the trick you observed 4 years ago involved a square cut piece of paper being rolled about a skunky smelling green herb, ultimately producing a round cylindrical object tapering off near it's ends. The lighter was then used to ignite the object. The combustion and smoke arising from such objects can have the side effect of one's inability to remember the point of a demonstration?
My apologies to all parties for poking fun this way. I meant no offense to anyone. I just couldn't resist the temptation.
gordo
Well-known member
As you well know Steve, in the context of this thread, AM and XM are the same. (chuckle)
best, gordo
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SteveAgfa
Well-known member
CTPPers...
I believe Gordon made reference to it earlier in this thread,
but perhaps you don't know that he was the primary editor
for the BRIDGS book on screening:
Publications - BRIDGS A Guide To Halftone Screening
Although I was a reviewer and contributor, Gordon did much
of the heavy lifting on this piece. It's a great resource, at a
great price.
Regards,
I believe Gordon made reference to it earlier in this thread,
but perhaps you don't know that he was the primary editor
for the BRIDGS book on screening:
Publications - BRIDGS A Guide To Halftone Screening
Although I was a reviewer and contributor, Gordon did much
of the heavy lifting on this piece. It's a great resource, at a
great price.
Regards,
Alith7
Well-known member
coming from a small shop, some of us don't have the money or the equipment to handle FM screening. So we do the best with what we have. PeterA, you are fortunate to be working in a shop that has the money and resources to stay with the higher technology.
I've been running a Xitron Xenith system for a few years with an AGFA Accento CTP. When we set up the Xenith they told me to set up my screens with the Euclidean+7 screen sets. What is the difference between that and standard Euclidean? I've never been able to get an answer other than "that's the best set you can use".
I don't like that answer.
Since we switched to the AGFA, we lost some of our resolution. we had to go from 2540 to 2400 on our tif files and I've been having problems with my screen sets ever since. would switching to a round dot help?
sorry for the questions, but you guys seem to know a lot about this and it's hard to find someone to help with this stuff.
I've been running a Xitron Xenith system for a few years with an AGFA Accento CTP. When we set up the Xenith they told me to set up my screens with the Euclidean+7 screen sets. What is the difference between that and standard Euclidean? I've never been able to get an answer other than "that's the best set you can use".
I don't like that answer.
Since we switched to the AGFA, we lost some of our resolution. we had to go from 2540 to 2400 on our tif files and I've been having problems with my screen sets ever since. would switching to a round dot help?
sorry for the questions, but you guys seem to know a lot about this and it's hard to find someone to help with this stuff.
gordo
Well-known member
The difference between 2540 to 2400 (2540dpi/100dpmm, 2400 dpi/96dpmm/) should not be significant unless you are working with prescreened (1 bit) files. 2540 is simply the metric/Euro standard.
Couldn't find any info on the Xitron website about their screening. I'm just guessing here but the + 7 might mean that the angles are offset by 7 degrees - you could check that yourself if you have a screen angle finder. If that's correct then that is one technique for minimizing what's called "single channel moiré"
best gordo
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